Although there are various pressure treatment methods for impregnating wood with active ingredients, the Bethel process is one of the better known. In the Bethel process, wood is dried so that its moisture content is substantially reduced. The wood is placed in a vacuum chamber to draw air from the wood. A mixture of water and active ingredients is then injected into the chamber under pressure. Pressures up to 250 pounds per square inch (psi) can be applied. The pressure is removed so that the wood is again subject to atmospheric pressure. The wood then is transferred to a kiln and dried to reduce the moisture content thereby leaving the active ingredient infused and imbedded in the wood.
The prior art of pressure treatment of wood has proven unsatisfactory in several respects. Most pressure treated wood is used outdoors and is exposed frequently to water, which is able to seep into the prior art pressure treated wood. The movement of water in and out of the wood causes two things to occur. First, the water dissolves any water soluble active ingredients and extracts those ingredients from the wood, thereby reducing the beneficial properties the ingredients may have imparted, such as rot prevention or flame retardance. Second, the water causes dimensional instability of the wood, which can take the form of splitting and cracking upon freezing.
An effective active ingredient commonly used for the pressure treatment of wood is Copper Chrome Arsenate (CCA), a heavy metal. The possibility of leaching has caused some persons to criticize the use of CCA.
The problem of leaching of active ingredients from pressure treated wood is recognized in the prior art, and attempts have been made to address the problem. One prior art attempt at a solution is to use polymeric binders to secure particles of an active ingredient to the wood. These polymeric binders typically use aminoplast curing agents that have the undesirable characteristic of generating formaldehyde. Formaldehyde has various undesirable characteristics, such as generating odors. Formaldehyde also is a suspected carcinogen.